In the present work, the issues of isotropic/anisotropic character of adsorption-induced deformation of carbon adsorbents were addressed. A simple model of microporous zones of carbon adsorbents was developed in terms of their structure determined by a raw material and activation conditions. The model made it possible to evaluate the number of micropores (~ 1020 g−1) and the number of microporous nanocrystals with micropores (denoted as an elementary microporous zone, EMZ) ~ 1011 g−1 for four carbon adsorbents differing in raw material and activation conditions. The equiprobable orientation of the EMZ in activated carbons nullifies the influence of the anisotropic structure of nanocrystallites on the changes in macroscopic dimensions of the adsorbents stimulated by adsorption. The assumption was confirmed by the dilatometry measurements of the CO2 adsorption-induced deformation for the commercial activated carbon Sorbonorit-4 granules, which were deposited parallel and perpendicular to the vertical axis of the dilatometer. The adsorption-induced strain isotherms did not depend on the orientation of the Sorbonorit-4 granules and exhibited the non-monotonic character. The initial contraction of the adsorbent was followed by its expansion with increasing pore filling. The contraction–expansion transition, as well as the contraction magnitude, were found to be temperature-dependent within the temperature range from 216.6 to 393 K. The compressibility and triaxial compression modulus of Sorbonorit-4 were evaluated over the temperature range from 216.6 to 293 K. The temperature dependences of both parameters were approximated by exponential functions. The triaxial compression modulus of Sorbonorit-4 decreased from 42 to 10 GPa and the compressibility increased by five times with a rise in temperature from 216.6 to 293 K.

This work is part of the effort at the Facility for Adsorbent Characterization and Testing (FACT lab) at the National Institute of Standards and Technology (NIST) to develop reference materials, reference data, and measurement best practices for adsorption metrology. High-pressure surface excess isotherms for CO2 adsorption on NIST Reference Material 8852 (ammonium ZSM-5) at 15 °C, 25 °C, and 35 °C are reported, expanding on a FACT Lab-organized international interlaboratory study on this sorbent/sorbate pair at 20 °C. The range of temperatures of the present study is of interest for many CO2 adsorption measurements and applications. Measurements were made using five different adsorption instruments, both manometric and gravimetric. Excellent agreement in the measured isotherms among the instruments was found. An empirical reference equation of the form, \({n}_{ex,ref}({P}_{eq})=\frac{c}{\{{1+\mathrm{exp}[\left(-\mathrm{ln}\left(P\right)+a\right)/b ]\}}^{b }} ,\) [nex,ref is surface excess uptake (mmol/g); Peq is equilibrium pressure (MPa); P is (Peq)/(1 MPa)]; a, b, c are constants] and the 95% uncertainty interval were determined for the isotherms at each of the temperatures. Lastly, the isosteric heat of adsorption is estimated from absolute isotherms derived from the surface excess reference data.

As a triatomic allotrope of oxygen, ozone has many industrial and consumer applications because of its high oxidation potential and disinfectant properties. Most of the ozone is currently produced by corona discharge reactors using high-purity oxygen as the feed gas, where the conversion to ozone is typically 6−15 wt%. Given the high operating costs associated with the unreacted oxygen, it is essential to recover the oxygen for recycling to the ozone generator. This paper aims for the first time to design, simulate and assess cyclic adsorption processes for recovering oxygen from a mixture of ozone and oxygen (O3:O2 = 10:90 wt%) produced by a commercial ozone generator. A 2-bed, 2-step cycle was first investigated in which ozone is selectively adsorbed at high pressure and then desorbed by dry air used as a sweep gas. However, it was found that the O2 purity in the recycle stream was too low because of the nitrogen build up from air. Thus, two additional cycles, namely a 3-bed, 3-step cycle incorporating an oxygen purge step and a 3-bed, 4-step cycle incorporating a vacuum blowdown step, were evaluated with the purpose of removing nitrogen from the bed after the desorption step. Both adsorption cycles achieved a satisfactory O2 purity of 98+% in the recycle stream with a sacrifice of O2 recovery and an increase in energy consumption, respectively. As a result, the adsorption processes devised in this study could serve to significantly reduce the oxygen consumption in industrial ozone generators.

A new non-experimental methodology based on a 3D Navier–Stokes (NS) computational fluid dynamics (CFD) model in lieu of experiments, is proposed for developing 1D axial pressure drop correlations for structured adsorbents with parallel channels. To demonstrate the methodology, a 1D correlation was developed by fitting and validating a Darcy–Weisbach (DW) type expression against the 3D-NS model. The methodology was further validated by contrasting the 1D-DW correlation against bench-scale experimental data obtained from a Catacel structured adsorbent with parallel triangular channels and against two 1D pressure drop correlations in the literature for parallel triangular channels. A wide range of velocities, pressures, channel dimensions and gas molecular weights were explored. To resolve the 1D-DW correlation, expressed in terms of a Darcy friction factor involving two fitting parameters f1 and f2, an analytic expression derived from the differential 1D-DW model was simultaneously regressed with all the results obtained from the 3D-NS model using air at 25 °C. Then predictions from the differential 1D-DW correlation, now solved numerically in COMSOL 5.2, were contrasted against those from the 3D-NS model for the same conditions using CO2 and He in addition to air at 25 °C. The 1D-DW correlation agreed well with the 3D-NS model, with the average and standard deviation of the average relative errors (AREs) being 1.79% \(\pm \) 1.89%. The 1D-DW correlation also showed good agreement with experiment for all outlet pressures and gases. The average and standard deviation of the AREs were 10.96% \(\pm \) 5.59%. Additionally, the 1D-DW correlation agreed well with the Boussinesq, and Shah and London correlations, with the average of the AREs relative to the 3D-NS model respectively being 2.00, 2.47 and 6.17%. These results validated the new non-experimental procedure for developing 1D axial pressure drop correlations from 3D CFD modeling in lieu of experiments. This new methodology is applicable to virtually any structured adsorbent shape with parallel channels and is especially advantageous where experiments might be problematic.

Simulation and design of adsorptive separation units demand accurate estimation of thermodynamic properties. Isosteric heat of adsorption as calculated from generalized Langmuir (gL) isotherm coupled with Clausius–Clapeyron expression for pure component and mixed-gas adsorption equilibria is presented in this work. The estimated isosteric heat of adsorption as functions of surface loading and composition is validated against the experimental data for various adsorption systems. Furthermore, the gL results are compared against classical Langmuir (cL) and Toth isotherm for pure components and with Ideal Adsorbed Solution Theory (IAST) for mixed-gas adsorption equilibria. The comparison highlights that gL outperforms cL and Toth for pure component adsorption and IAST for mixed-gas adsorption, and gL reliably captures the loading dependence and the composition dependence for isosteric heat of adsorption.

Alumina catalysts are frequently used in refineries for the hydrotreatment of heavy petroleum fraction that are enriched in asphaltenes. The transport of real and model asphaltenes molecules through powder and alumina’s extrudates treated or not at 150 °C to remove or not surface-adsorbed water was studied. The kinetics and isotherms of adsorption at 298 K were obtained by the solution depletion method. Calorimetric experiments were also investigated. The kinetic is faster on the powder than on the alumina extrudates where the equilibrium is reached after 24 h (against 1 h for the powder) due to mass transfer limitation. The capacity of adsorption of model asphaltenes on untreated powder and extrudates is comparable around 1.1 and 1.2 mg.m− 2 and increases with the heat treatment due to water removal. Both adsorption strength and capacity of real asphaltenes on alumina is lower compared to the model asphaltene molecule which could be explained by the strong interaction between the acidic function of the model molecule and the alumina surface. The calorimetric study in absence of alumina shows the dimerization of model asphaltene molecules. In presence of alumina, the enthalpy of adsorption of model and real asphaltenes on alumina is determined. The enthalpy of adsorption of model asphaltenes on treated powder is higher than on untreated powder meaning that more energetic sites are available (probably due to the release of water-occupied sites) and the curve obtained for treated powder suggests different adsorption sites. The enthalpy of adsorption of model asphaltene is higher for the treated extrudates but these results must be taken carefully because the kinetic of adsorption is very slow (24 h) for extrudates. The effect of flow rate was studied by saturating an extrudate column with model asphaltene molecules. The adsorption increases as the flow rate decreases which could be explained by higher friction in the macropores leading to the release of weakly retained asphaltenes as the flow rate increases or by less intermediate pore blocking by asphaltenes as the flow rate and thus the pressure increases. This study shows that the transport of asphaltenes through porous alumina supports is a complex process depending on many parameters.

Microporosity and structure of a set of activated carbons was studied by combination of N2 and CO2 adsorption, Transmission Electron Microscopy (TEM), X-ray diffraction and scattering and multiwavelength Raman spectroscopy. It is shown that correlations between measured parameteres may be established for a given set of activated carbons, most often obtained from a same precursor. Comparison of results of TEM images processing and of Small-angle scattering with adsorption data suggests that super-micropores (0.7–2 nm) are highly variable in shape and strongly deviate from the ideal slit pore model. These pores are likely located in between disordered continuous graphene stacks. It is shown that Small-angle scattering is mostly caused by supermicropores; contribution of other types of porosity is of secondary importance. For a set of carbons with similar structure, a reasonable correlation between Guinier radii and pore width obtained from N2 adsorption can be found; however, the reason for the observed offset between the data sets remain uncertain. Sensitivity of the Raman scattering to atomic scale processes leads to poor or unclear correlations between the spectroscopic and structural data, although some notable exceptions are noted.

The non-isothermal adsorption/desorption of gas in a cylindrical column filled with adsorbent particles has been considered theoretically. The model is based on Langmuir theory for equilibrium adsorption. Using the Heaviside operational method the analytical solutions for kinetics of changes of concentration of adsorbate in the inter- and intra-particle spaces were obtained. The numerical calculations of gas adsorption at different temperature of the gas flow (Ta = 273–373 K) followed by desorption by the flow of inert gas through the column at Td = 673 K were done. The results revealed the rather complex interrelations between spatial–temporal dependencies in distribution of concentrations of adsorbed gas inside adsorption column and temperatures of inlet gas.Graphical Abstract

In this study four kinds of biochars were prepared from the KOH modified biomass. As the carbon precursors there was used the sawdust from the following trees: oak, hornbeam, apple and cherry. The physicochemical properties of the materials were characterized by the N2 adsorption, scanning electron microscopy, thermal analysis (TG, DTG and DTA), infrared spectroscopy, and the Boehm’s titration method. Moreover, pHpzc (the point of zero charge) was determined. The adsorption capacity and the temperature-programmed desorption of ammonia were also studied. The obtained activated biochars were characterized by the large specific surface area (672 to 912 m2/g) and the total pore volume (0.30 to 0.4 cm3/g) as well as the well-developed microporous structure (85–97%). These observations were also confirmed by the SEM analysis. The maximum NH3 adsorption capacity of the activated biochar was determined to be 3.05 mmol/g. These results prove that the sawdust of various origins is appropriate to prepare a cost-effective, environmentally friendly biochar.

The cap-removal conditions of chemical vapor deposition-prepared single wall carbon nanotube (SWCNT) bundles were optimized via air oxidation at different temperatures.We determined specific surface areas (SSAs) of the SWCNT oxidized at different temperatures from N2 adsorption isotherms with the aid of the subtracting pore effect (SPE) method enabling evaluation of the SSA of microporous carbons without overestimation. The oxidation of SWCNT at 773 K gave the maximum SSA of 1840 m2 g−1. We chose the oxidation temperature of 773 K for the optimum oxidation for the cap-removal without serious crystallinity damages. The SSAs from the SPE method of the SWCNT bundles before and after oxidation at 773 K almost coincided with the geometrically evaluated SSAs of SWCNT with and without caps, confirming the selective removal of caps from SWCNT through the oxidation at 773 K. TEM and Raman spectroscopic examination guaranteed the well-crystalline state of the SWCNT bundles oxidized at 773 K, as suggested from N2 adsorption measurements.

This paper is dedicated to the memory of Academician Mikhail Mikhailovich Dubinin (1901–1993), the eminent Russian scientist who made an outstanding contribution to adsorption science—one of the most important subdisciplines of physical chemistry. A brief account of his life, professional carrier, and scientific achievements is presented.

Adsorption of polyions, strongly charged poly(N-ethyl-4-vinylpyridinium) cation (P4VP) weakly charged poly(allylammonium) cation (PAH) and weakly charged poly(acrylate) anion (PAA) on TiO2 surface was investigated. The selected polyions have different functional groups and degree of polyion ionization, which affects the interactions with the TiO2 surface. The effect of polyion adsorption on the inner surface potential and the electrokinetic potential were measured and analyzed. The inner surface potential of TiO2 was measured by means of single crystal (SCr) electrodes with two different crystal orientation (001) and (100). The electrokinetic potential of titania particles was obtained from electrophoretic measurements. In suspensions, uncoated TiO2 particles tend to aggregate and aggregation is reduced after polyion adsorption. The process of aggregation was monitored measuring the size of the particles. Since the surface charge of TiO2 and the degree of polyion ionization depend on the composition of the solution, namely on pH and ionic strength, the adsorption of polyions on TiO2 also depends on these parameters. The contribution of the surface charge and degree of polyion ionization of the selected polyelectrolytes to the rate of aggregation of TiO2 particles was examined. It was found that in the case of weak polyelectrolyte the degree of polyanion ionization plays significant role in stabilization of titania suspensions.

The aim of this study is to unravel the influence of single, double and triple bonds in hydrocarbons on the mechanisms of adsorption on zeolites. Therefore, the adsorption of the C2 hydrocarbons ethane, ethene and ethyne on different adsorbents is studied by adsorption calorimetry. As adsorbents pure sodium LTA (NaA) and FAU (NaX) zeolites and calcium exchanged CaNaA and CaNaX zeolites are used. Based on experimental loadings and heat of adsorption, the influence of the number and distribution of cations on different cation positions are discussed in detail. With increasing degree of exchange the increasing number of Ca2+-cations introduce energetically more valuable adsorption sites into the zeolites. On the other hand, the decreasing total number of cations has a negative effect on saturation loading. The impact of these opposing effects and the different occupation of cation positions on the interactions and mechanisms occurring are discussed. The loading increases from ethane to ethene to ethyne and shows higher values on FAU compared to LTA. In terms of interactions, due to the single bond, in ethane only dispersion and induction interactions are formed. In ethene and ethyne due to the double and triple bonds, respectively, additional quadrupole cation and π-interactions occur. In this study, for the first time the formation of a π-complex with Ca2+-cations at position I in LTA and at positions II, III, and III’ in FAU is demonstrated. For ethyne, additional π-complex formation with Na+-cations on the identical positions is also detected, which was previously unknown in literature.

Dual reflux pressure swing adsorption is a peculiar application of pressure swing adsorption with relevant separation potential. In a proper range of operating parameters, high separation performances are achieved in many applications, including complete separation in the case of binary mixtures. In this work, a new representation of the design parameters suitable for complete separation based on the semi-analytical solution of the corresponding Equilibrium Theory model is proposed for the four basic process configurations. Namely, given feed position, feed composition, and adsorbent separation selectivity, the combinations of the remaining process parameters ensuring complete separation are identified in a 3D plot. Furthermore, the same model equations have been solved numerically to explore conditions of incomplete separation, where the semi-analytical solution is not available. In particular, the sensitivity of the separation performances of each configuration to the pressure ratio has been explored. More specifically, given the region of operating conditions suitable for complete separation, selected operating conditions outside this region have been explored aimed to recover/improve the separation quality. Even though the different configurations exhibit different behaviors, the general dependence of the product purity upon the pressure ratio is quite limited and only minor improvements can be obtained with a few exceptions.

Single-walled carbon nanotube (SWCNT) based materials are considered as one of the most promising adsorbents for adsorption-based technology for methane storage. Molecular dynamic simulations are employed to investigate the assembling of single-walled carbon nanotubes into an array via toluene coordinator-molecules. The smallest number of toluene molecules sufficient to maintain the ASWCNT + nC7H8 supramolecular structure is defined. The potential, kinetic and total energies of the simulated system plotted as a function of toluene molecules are used to evaluate the stability condition for the supramolecular structure. Methane adsorption onto the model ASWCNT + nC7H8 adsorbent is simulated to assess the contributions of toluene molecules to its adsorption capacity. The radial and angular probability density distributions of methane molecules in the ASWCNT + nC7H8 adsorbent reveal the most probable location of adsorbed methane near the SWCNT walls and toluene molecules as centers of adsorption. A SWCNT-based adsorbent is prepared by the saturation with toluene and subsequent two-step regeneration with a slow-rate increase in temperature. The content of toluene of 5wt.% in the adsorbent thus obtained is evaluated from a gain in weight observed at 338 K. Methane adsorption on the initial SWCNT and SWCNT + 5%C7H8 adsorbents is measured at 178 and 273 K up to 101 kPa. A comparison of the initial and differential molar heats of methane adsorption on the initial SWCNT and ASWCNT + nC7H8 adsorbents points to the role of toluene molecules as additional centers of adsorption. We estimate the contribution of toluene molecules to the porosity of the ASWCNT + nC7H8 adsorbent from the methane adsorption data.

Nanoporous carbon sorbents modified with oligomers of lactic and glycolic acids were synthesized without the use of catalysts and organic solvents. Their synthesis includes impregnation of the sorbent with a 50% aqueous solution of hydroxy acid followed by multistep thermal treatment. Physicochemical properties of the modified carbon sorbents were studied using low-temperature nitrogen adsorption, scanning electron microscopy, elemental, thermal and spectrophotometric analyses, nuclear magnetic resonance spectroscopy and viscometry. Modification of the carbon sorbent with oligomers of hydroxy acids is accompanied by the appropriate deterioration of its textural characteristics and substantial increase in the oxygen content. The deposited oligomers of glycolic and lactic acids are distributed locally on the carbon surface. It was shown that the sorbent modified with glycolic acid oligomer was more hydrophilic. It exhibited a higher adsorption capacity with respect to gelatin as compared to the sorbent modified with lactic acid oligomer. Differences were found in the biological activity of modified samples toward pathogenic bacteria Pseudomonas aeruginosa, Enterococcus faecalis, Escherichia coli, yeast-like fungi of the Candida genus, and bacterial-fungal Staphylococcus aureus + Candida albicans association. The higher biological activity of the modified carbon sorbents in comparison with the initial sample was attributed to the acid–base properties of deposited oligomers and their ability to biodegrade in an aqueous medium.Graphical abstract

This comment seeks to establish a relation between two definitions of the pore volume of a microporous crystalline material. According to the first definition based on the Gurvich rule, the volume of the pores can be estimated from the saturated amount of vapour adsorbed, using the bulk liquid density of adsorbate as the conversion factor. The second definition is based on a purely geometric consideration of the porous space. With argon as the adsorbate and all-silica zeolite structures from the International Zeolite Association (IZA) database as the model adsorbents, we generate adsorption data using Grand Canonical Monte Carlo simulations and structural characteristics of the materials from the Poreblazer PB4.0 software. Under confinement in zeolitic pores, adsorbed argon forms structures very different from the liquid-like configurations. However, the pore volumes of these materials obtained from the Gurvich may deviate positively or negatively from the reference geometric value. Considering simply the geometric features of the materials, such as the pore volume itself or the pore size distribution, it proved to be difficult to anticipate how the volume from the Gurvich rule would deviate from the geometric volume for a particular structure. Overall, volume from the Gurvich rule agrees with the geometric volume within 25% error for 82% of the structures from the IZA database. As an additional outcome of this study, we provide a comprehensive database of textural characteristics and simulated argon adsorption data for all-silica zeolites, which can be used as reference values for the assessment of the quality of the microporous samples of all-silica zeolites in future experimental studies.

The adsorption excess isotherms of ethanol–water and propanol–water mixtures are studied on a series of carbon molecular sieves with well-separated micro- and mesoporosity at 298.15 K. The preferential adsorption of one component from a mixture is measured by using vibration densitometry for the concentration analysis. Microcalorimetrically measured enthalpies, which are released upon immersion of the carbon materials in the binary mixtures, complement the adsorption excess data. It is shown that (i) density measurements are well applicable for studying liquid-phase adsorption, (ii) liquid-adsorption isotherms are sensitive to smallest chain length differences of the adsorptives, (iii) the calculated separation diagrams depend strongly on the assumptions about the adsorbed phase, and (iv) the combined determination of gas, vapor and liquid adsorption isotherms and immersion enthalpies offers advantages for the analysis of complex adsorption systems.

The adsorption of chloroxylenol and chlorophene on halloysite-carbon composites was investigated in batch and flow systems. The synthesis of halloysite-carbon composites through two different methods was performed with microcrystalline cellulose as carbon precursor. The obtained halloysite-carbon composites were characterized by SEM/EDS analysis, the low-temperature nitrogen adsorption/desorption methods, and infrared spectrometry (FT-IR). The SEM/EDS analysis and FT-IR spectra confirmed the presence of carbon on the surface of the halloysite. On the basis of the measurement results in the batch system, the two composites with the best adsorption properties for both adsorbates were chosen for measuring the flow system (using the inverse liquid chromatography). Removal efficiency was equal to 92.26 and 81.36%. It was obtained for chloroxylenol on HNT-m 800 and HNT-Zn 500, respectively. For chlorophene, the removal efficiency had the value of 78.79 and 77.87% on HNT-m 800 and HNT-Zn 800, respectively. Adsorption parameters of chloroxylenol and chlorophene were determined with inverse liquid chromatography methods: the adsorption equilibrium constants were determined with the peak division method and the adsorption capacity of the adsorbents was determined with the breakthrough curve method. Maximum adsorption capacity for the adsorption of chloroxylenol on HNT-m 800 was 5.48 mg·g−1 and on HNT-Zn 500 its value was 2.77 mg·g−1. For the adsorption of chlorophene on HNT-m 800 the value was 4.44 mg·g−1 and on HNT-Zn 800–2.5 mg·g−1. Halloysite-carbon composites can be successfully used as effective adsorbents for removing chloroxylenol and chlorophene from solutions in the flow system.

AbstractThis work studies the influence of nitrogen doping of carbon nanotubes (CNTs) and few-layer graphene nanoflakes (GNFs) on the adsorption of organic solvent vapours. The synthesized materials (CNTs, GNFs and N-doped counterparts) were thoroughly characterized by simultaneous thermal analysis, low temperature nitrogen physisorption and X-Ray photoelectron spectroscopy. The basic regularities of the adsorption of organic solvent vapours were studied using acetone, ethyl acetate, acetic acid and toluene. The dependence of the maximum adsorption capacity of CNTs, GNFs and N-doped counterparts on the dipole moment of adsorbate was investigated. The dependencies of the isosteric heat of adsorption on the degree of surface coverage for various pairs of adsorbate-adsorbent were obtained and the average values of the heat of adsorption were calculated. It was found that introduction of nitrogen into the structure of CNTs and GNFs significantly increases the heat of acetic acid vapour adsorption.